HYALONEMA (THALLONEMA) GEMINATUM. 391 
tine. The distal ray is straight and 178-270 » long, most frequently about 
250 w. It usually ends in a rather stout and sharp-pointed terminal cone (Plate 
103, figs. 59-62). Exceptionally the tip is rounded (Plate 103, fig. 58). The 
basal thickness of the ray is 7-11 ». Apart from small parts of it at the basal 
and distal ends, the ray is quite densely spined. The spines are for the most 
part nearly straight. Those arising from the middle-part of the ray are of 
considerable size, up to 30 4 and more long. The maximum thickness of the 
distal ray, together with the spines, is 832-46 ». The lateral rays are conical, 
blunt-pointed, spined, and 28-43 yu long. 
The small, probably canalar pinules are pentactine. The distal ray is 
straight, usually 90-110 » long, and 4—6 » thick at the base. Its base and its 
sharp-pointed distal end-part are smooth; its middle-part bears very large, 
strongly divergent, sparse spines, which are curved, concave towards the tip of 
theray. The largest spines are found in the proximal part of the spine-bearing 
region. The lateral rays are conical and 32-60 u long, generally 40-50 up. 
The hypodermal and hypogastral pentactines have smooth, straight, and 
very blunt, conical rays. The proximal ray is 0.2-1 mm. long, and 7-22 u 
thick at the base. The lateral rays are 160-450 u long. 
The hexactine megascleres are generally 0.8-1.8 mm. in diameter and have 
smooth, blunt, conical rays 15-45 » thick at the base. 
The superficial and choanosomal amphioxes are usually fairly straight, 
rarely markedly curved, and 0.4-2.6 mm. long. The shorter ones, that is those 
under 1 mm. in length, are distinctly centrotyle with a tyle 1-4 u more than 
the adjacent parts of the spicule in transverse diameter. The medium ones, 
that is those 1-1.5 mm. in length, have only a very insignificant tyle, not more 
than 1.5 » thicker than the adjacent parts of the spicule. In the large ones, 
that is those over 1.5 mm. in length, there is hardly a trace of a central tyle. 
The microhexactines (Plate 103, figs. 39-48) are 37-120, usually 53-100 u 
in diameter (maximum diameter). Most of them have equal rays; in some 
two opposite rays are longer than the other four. The rays are 1.8—2.2 » thick 
at the base, conical, and finely pointed. The proximal part, about the half of 
the ray, is straight, the distal part curved. This curvature is either uniform, 
or, more frequently, at the point where the proximal straight part passes into 
the distal curved part greater than elsewhere. The total curvature is such that 
the directions (tangents) of the proximal and distal end-parts of the ray usually 
enclose an angle of about 120°. The rays bear very numerous, exceedingly 
minute spines, which give them the appearance of being rough. 
